Satellite positioning systems such as GPS (Global Positioning System), Galileo, GLONASS, QZSS, Compass, IRNSS and others make use of “spread-spectrum” modulated navigation signals. These signals essentially carry pseudo-random codes made up of numerical sequences which repeat periodically, the main function of which is to permit Code Distribution Multiple Access (CDMA) and the provision of an accurate measurement of the propagation time for the signal transmitted by the satellite. Satellite positioning signals may incidentally also carry useful data.
In the case of GPS, the navigation signals are transmitted in the L1 frequency band, centred on 1575.42 MHz, and L2 frequency band, centred on 1227.6 MHz. The L5 band, centred on 1176.45 MHz, will be added during the modernisation of GPS. The satellites of the Galileo constellation will transmit in the bands: E2-L1-E1 (the median band portion L1 being the same as that for GPS), E5a (which, according to Galileo nomenclature, is the L5 band intended for GPS), E5b (centred on 1207.14 MHz) and E6 (centred on 1278.75 MHz).
The navigation signals are formed by modulating the central (carrier) frequencies. Various modulation schemes have already been established or are at least under consideration for creating navigation signals. In order to ensure interoperability and compatibility between the GPS and Galileo systems, the United States of America and the European Union have agreed upon certain points relating to signal modulation schemes in the L1 band, which is used by both systems. More details about the proposed modulation schemes may be obtained from the publication “MBOC: The New Optimized Spreading Modulation Recommended for GALILEO L1 OS and GPS L1C”, Hein et al., Inside GNSS, May/June 2006, pp. 57-65.
One of the modulation schemes selected as a candidate for modulating the Galileo OS L1 signal is known by the name “TMBOC modulation”. This type of modulation has moreover already been selected for the L1C GPS signal. The TMBOC spreading waveform modulating the carrier may be described as an alternating succession of segments of a first BOC(n2,m) waveform and of segments of a second BOC(n1,m) waveform, with n1>n2. “BOC” denotes a double offset carrier modulation, the abbreviation standing for “Binary Offset Carrier”.
In general, BOC(n,m) is a time function t defined by:BOC(n,m)(t)=Cm(t)·sign[sin(2πfsct)],  (1)where Cm(t) is a pseudo-random code of a chip rate m×1.023 Mcps assuming the values +1 or −1 and fsc is the frequency n×1.023 MHz. One condition applying to n and m is that the ratio 2n/m is integral. The TMBOCm(n1,n2) spreading waveform is defined by:
                                                        TMBOC              m                        ⁡                          (                                                n                  1                                ,                                  n                  2                                            )                                ⁢                      (            t            )                          =                  {                                                                                                                                        C                        m                                            ⁡                                              (                        t                        )                                                              ·                                          sign                      ⁡                                              [                                                  sin                          ⁡                                                      (                                                          2                              ⁢                              π                              ⁢                                                                                                                          ⁢                                                              f                                                                  n                                  ⁢                                                                                                                                          ⁢                                  2                                                                                            ⁢                              t                                                        )                                                                          ]                                                                              ,                                                                                                  if                    ⁢                                                                                  ⁢                    t                                    ∈                                                                                                                                                                        C                        m                                            ⁡                                              (                        t                        )                                                              ·                                          sign                      ⁡                                              [                                                  sin                          ⁡                                                      (                                                          2                              ⁢                              π                              ⁢                                                                                                                          ⁢                                                              f                                                                  n                                  ⁢                                                                                                                                          ⁢                                  1                                                                                            ⁢                              t                                                        )                                                                          ]                                                                              ,                                                                                                  if                    ⁢                                                                                  ⁢                    t                                    ∈                                                                                        (        2        )            where fn1=n1×1.023 MHz, fn2=n2×1.023 MHz, where S1 is the union of the “BOC(n1,m)” segments and S2 the union of the “BOC(n2,m)” segments, S1 and S2 being complementary on the time axis, and where Cm(t) is the pseudo-random code of the signal at a chip rate m×1.023 Mcps and assuming the values +1 or −1. For the GPS L1C and Galileo OS L1 signals, m=1, n2=1 and n1=6 will occur as applicable. The ratio between the length of the “BOC(1,1)” segments and the length of the “BOC(6,1)” segments determines how the power of the signal is distributed between its two components.
Another candidate modulation scheme for modulation of the Galileo OS L1 signal is known by the name “CBOC modulation”. The CBOC spreading waveform modulating the carrier is a linear combination of a first BOC(n2,m) waveform and a second BOC(n1,m) waveform. In this case, a CBOCm(n1,n2) waveform may be written:CBOCm(n1,n2)(t)=V·BOC(n2,m)(t)+W·BOC(n1,m)(t),  (3)where V and W are real parameters defining the relative weighting of components BOC(n2,m) and BOC(n1,m). In the case of a CBOC waveform, the two BOC components carry the identical pseudo-random code. If this modulation is selected for Galileo OS L1, m=1, n1=6 and n2=1 will apply.
In order to determine the propagation time of the signal transmitted by a satellite (the pseudo-distance) in a receiver, the method of receiving the signal comprises a correlation stage. It is well known in the technical field to correlate the waveform modulating the radionavigation signal with local replicas of this modulation waveform. The modulation waveform has an at first sight unknown phase which must be determined to calculate the position of the receiver. The method normally proceeds iteratively and begins with an initial estimate of the unknown phase of the modulation waveform. A prompt local replica of the modulation waveform is then generated in the receiver, i.e. a copy of the modulation waveform, the phase of which corresponds to the estimate, which is then correlated with the modulation waveform. In parallel, one or more offset local replicas of the modulation waveform are generated, i.e. one or more copies of the modulation waveform, the phase of which is early or late relative to the estimate. This or these offset local replicas are also correlated with the modulation waveform. The results of these correlations are then used to improve the estimate of the phase of the modulation waveform. The method is then reiterated until the phase has been determined with sufficient precision.
European patent application EP 1 681 773 describes this receiving method in the case of a CBOC type modulation waveform. The incoming signal modulated by a CBOC waveform and a local replica of this CBOC waveform are thus correlated. This solution involves generating a replica CBOC in the receiver. It is therefore necessary to implement four-level quantisation at the correlator input, which requires an at least 2-bit architecture. The same patent application also mentions a second method in which a correlation is performed between the incoming signal and a local replica of the first BOC component and another correlation between the incoming signal and a local replica of the second BOC component. The results of the two correlations are then combined. In this second method, the local replicas are one-bit, which may be considered advantageous relative to the first solution. The price to be paid is a doubling of the number of correlation operations in comparison with the first solution, all other things being equal.
French patent application 06 05551 presents an improved method and improved receiver for receiving a CBOC signal having a component BOC(n1,m) and a component BOC(n2,m), with n2<n1. In order to perform correlation between a local waveform and the CBOC waveform broadcast by the satellites, over a time interval of duration T, this application proposes generating the local waveform as a binary waveform, formed over said time interval of an alternating succession comprising at least one segment of a BOC(n1,m) waveform and at least one segment of a BOC(n2,m) waveform, the at least one segment BOC(n1,m) having a total duration of αT, α being strictly between 0 and 1, the at least one segment BOC(n2,m) having a total duration (1−α)T. In particular, this method does not involve a waveform with more than two levels and neither does it require a higher number of correlators.
FIG. 3 shows the simplified diagram of a receive channel of a receiver capable of implementing the method described in FR 06 05551. It will be noted that the same local binary waveform sLOC is used to carry out the various correlations.
On implementation of the method described in FR 06 05551, it is observed, in particular when m=1, n1=6 and n2=1, that if α increases, i.e. if the proportion of component BOC(6,1) is increased to the detriment of the proportion of component BOC(1,1) in the local waveform sLOC, degradation of the ratio C/N0 (ratio of the power C of the carrier to the spectral noise density N0) becomes more significant, so making signal reception more difficult. Degradation of the C/N0 ratio as a function of the value of the parameter α is shown in FIG. 4 for two types of CBOC modulation waveforms (one with one eleventh of the total power in component BOC(6,1), the other with two elevenths, this power distribution being mentioned by way of example). On the other hand, if α increases, an increase in synchronisation performance (“tracking performance”) and better resistance to multipath effects are also observed.